The repair of membrane damage induced by lipid peroxidation may occur by removal of the fatty acid hydroperoxide from the phospholipid and the insertion of a new fatty acid by an acyl transferase. The hydroperoxides may then be used by the microsomal hydroperoxide peroxidase to oxidize other biological compounds. Experiments using specific metabolic inhibitors are described that may distinguish the peroxidase activity from the other oxidative enzymes present in the microsomal fraction. Induction experiments, using phenobarbital, 3- methyl cholanthrene and ethanol, are described in an additional effort to differentiate the peroxidase activity from the expected increases in the cytochrome P450 and P448 drug metabolizing systems. The microsomal membrane will be solubilized using emulgen 911, a nonionic detergent. The peroxidase can be separated from the other cytochromes and enzymes by column fractionation and electrophoretic separation. The involvement of the hydroperoxide peroxidase in the oxidative metabolism of ethanol, thyroxine, steroids and drugs is critically examined. Finally, these metabolic systems are to be examined in the mother, placenta, fetus and newborn rats to evaluate the effect of externally administered agents on the development of the peroxidase and the drug metabolizing enzymes.